1. Field of the Invention
This invention relates to a closed loop single mixed refrigerant process wherein the capacity of the process can be increased by adjusting the temperature of the liquefied fluid material produced in the process.
2. Brief Description of the Prior Art
Because of its clean burning qualities and convenience, natural gas has been widely used in recent years. Many sources of natural gas are located in remote areas which are not conveniently available to any commercial markets for the gas. When pipelines are unavailable for transportation of the natural gas to a commercial market, the produced natural gas is often processed into a liquefied natural gas (LNG) for transport to market. One of the distinguishing features of an LNG plant is the large capital investment required for the plant. The liquefaction plant is made up of several basic systems including gas treatment to remove impurities, liquefaction, refrigeration, power facilities and storage and ship loading facilities. The cost of these plants can vary widely, but generally the cost of the refrigeration portion of the plant can account for up to 30% of the cost. LNG refrigeration systems are expensive because considerable refrigeration is necessary to liquefy the natural gas. A typical natural gas stream may be at a pressure from about 250 psig (pounds per square inch gauge) to about 1500 psig at temperatures from 40 to about 120.degree. F. The natural gas, which is predominantly methane cannot be liquefied by simply increasing the pressure on the natural gas as is the case with heavier hydrocarbons used for energy purposes. The critical temperature of methane is -82.5.degree. C. (-116.5.degree. F.) which means that methane can only be liquefied below that temperature regardless of the pressure applied. Since natural gas is commonly a mixture of gases, it liquefies over a range of temperatures. The critical temperature of natural gas is typically between about -121.degree. F. and about -80.degree. F. Typically, natural gas compositions at atmospheric pressure will liquefy in the temperature range between about -265.degree. F. and about -247.degree. F. Since refrigeration equipment represents such a significant part of the LNG facility cost, a considerable effort has been made to reduce refrigeration costs.
Various refrigeration cycles have been used to liquefy natural gas, with the three most common being the cascade cycle which uses multiple single component refrigerants and heat exchangers arranged progressively to reduce the temperature of the gas to liquefaction temperature, the expander cycle which expands gas from a high pressure to a low pressure with a corresponding reduction in temperature and multi-component refrigeration cycles which use a multi-component refrigerant and specially designed heat exchangers to liquefy the natural gas.
Natural gas is also liquefied in many instances to enable the storage of natural gas at locations near a demand for the natural gas, for instance, in heavily populated residential areas where there may be a greater need for natural gas during winter months than can be met by the available pipeline system. In such instances, liquefied natural gas may be stored in tanks, underground storage cavities and the like so that it can be available for use during the peak load months. The plants used to liquefy such gas for such storage may be somewhat smaller than those used to liquefy natural gas at remote locations for shipment to markets and the like.
Other gases are also liquefied but with somewhat less frequency. Such gases may be liquefied by the processes discussed above.
Previously, substances such as natural gas have been liquefied by processes such as shown in U.S. Pat. No. 4,033,735, issued Jul. 5, 1977 to Leonard K. Swenson, and U.S. Pat. No. 5,657,643, issued Aug. 19, 1997 to Brian C. Price, both of which are hereby incorporated in their entirety by reference. In such processes, a single mixed refrigerant is used. These processes have many advantages over other processes such as cascade systems in that they require less expensive equipment and are less difficult to control than cascade type processes. Unfortunately, the single mixed refrigerant processes require somewhat more power than the cascade systems.
Cascade systems such as the system shown in U.S. Pat. No. 3,855,810, issued Dec. 24, 1974 to Simon et al, basically utilize a plurality of refrigerant zones in which refrigerants of decreasing boiling points are vaporized to produce a coolant. In such systems the highest boiling refrigerant, alone or with other refrigerants, is typically compressed, condensed and separated for cooling in a first refrigeration zone. The compressed, cooled, highest boiling point refrigerant is then flashed to provide a cold refrigeration stream which is used to cool the compressed highest boiling refrigerant in the first refrigeration zone. In the first refrigeration zone, some of the lower boiling refrigerants may also be cooled and subsequently condensed and passed to vaporization to function as a coolant in a second or subsequent refrigeration zone and the like. As a result, the compression is primarily of the highest boiling refrigerant and is somewhat more efficient than when the entire single mixed refrigerant stream must be compressed. As noted, however, such processes require more expensive equipment.
In view of the reduced equipment cost and reduced difficulty of control with a single mixed refrigerant process, a continuing search has been directed to the development of such a process where the power requirements are reduced and wherein greater process flexibility is available.